1. Field of the Invention
The present invention relates to an optical add/drop multiplexer and an optical network system, and in particular to an optical add/drop multiplexer dropping and inserting (adding) a light of a specific wavelength with respect to a WDM signal light and an optical network system utilizing the same.
2. Description of the Related Art
In recent years, data communication demands have explosively increased mainly for the Internet traffic, so that increasing capacity and range of backbone networks and metro area networks has been required. On the other hand, services for users have been diversified into various kinds, so that realization of an economical network having a high reliability and flexibility has been required.
Specifically, since an optical transmission network forms a core of an infrastructure of an information and telecommunications network, further sophisticating and widening the service areas are desired, so that the development is on a rapid progress oriented to an information-driven society as expected to further advance.
As a main technology of a large-capacity optical transmission system, a WDM (Wavelength Division Multiplexing) technology has been used. The WDM technology is a method by which a plurality of optical wavelengths are transmitted with a single optical fiber by multiplexing lights of different wavelengths.
In a prior art WDM apparatus, an optical add/drop multiplexer for dropping wavelengths (of light) from a WDM transmission line and for multiplexing (inserting) wavelengths into the WDM transmission line by fixed wavelengths was generally a POADM (Passive Optical add/drop multiplexer) or an ROADM (Reconfigurable OADM) capable of variously dropping and adding fixed wavelengths.
However, in order to realize a network having a high reliability and flexibility, as a technology capable of dropping wavelengths and adding wavelengths with variable ones, namely as an optical add/drop multiplexer capable of dynamically changing the wavelengths to be dropped and/or added, DOADM (Dynamic Optical add/drop multiplexer) has been introduced.
As an example of such a DOADM apparatus, there is an optical add/drop multiplexer mainly used for a metro access region (e.g. 100 km ring) using an optical coupler and a wavelength selective filter when dropping arbitrary wavelengths from a WDM transmission line, and using an optical coupler and a reject/add filter when inserting arbitrary wavelength into the WDM transmission line (see e.g. non-patent document 1).
FIG. 12 shows an arrangement of such a prior art optical add/drop multiplexer 900. The optical add/drop multiplexer 900 is composed of an optical branching portion 910 having two optical couplers 911, 912 and a plurality of wavelength selective filters 913_1-913—n, and an optical inserting portion 920 having a reject/add filter 921, an optical coupler 922, and a plurality of inserting wavelength optical lasers 923_1-923—m. 
In operation, a WDM signal light 10 inputted to the optical add/drop multiplexer 900 is branched into two by the optical coupler 911 of the optical branching portion 910, to be provided to the optical coupler 912 in a dropping direction and to the reject/add filter 921 in a through direction. While the same wavelengths are multiplexed in each of two lights branched by the optical coupler 911 in this case, the light in the through direction will be referred to as a through wavelength 30 in order to facilitate later descriptions.
The light inputted to the optical coupler 912 is branched to be provided to the wavelength selective filters 913_1-913—n, and a drop wavelength 20 of a wavelength selected by each of the wavelength selective filters 913_1-913—n is outputted from the optical branching portion 910 to the outside of the optical add/drop multiplexer 900.
On the other hand, an add wavelength 40 inputted from the outside of the optical add/drop multiplexer 900 to the optical inserting portion 920 is converted into a predetermined wavelength in each of the inserting wavelength optical lasers 923_1-923—m, and then multiplexed by the optical coupler 922 to be provided to the reject/add filter 921.
Since the add wavelength 40 is included in the WDM signal light 10 or the through wavelength 30 provided from the optical coupler 911 of the optical branching portion 910, this add wavelength 40 is terminated in the reject/add filter 921 where the add wavelength 40 provided by the optical coupler 922 is multiplexed with the through wavelength so that a WDM signal light 50 is outputted.
It is to be noted that in the event of drastic optical power attenuation as a result of power branching by the optical coupler 911, at least one of optical amplifiers 915 and 914 has only to be inserted as shown in FIG. 12.
A general reject/add filter will now be described referring to FIGS. 13A and 13B. FIG. 13A shows a basic arrangement (single stage arrangement) of a reject/add filter. The reject/add filter 1 shown in FIG. 13A has a common port P1, an insert port P2, and a reflect port P3.
FIG. 13B shows an inner structure of the reject/add filter 1 of FIG. 13A. Among the common port P1, the insert port P2, and the reflect port P3, each of which is a terminal portion of the optical fiber F1, there are provided two lenses L1 and L2 sandwiching a multilayer film filter F. It is to be noted that the multilayer film filter F is occasionally called a thin film filter (TFF).
As shown in FIG. 13B, supposing that the multilayer film filter F transmits only the lights of wavelengths λ1-λ4, when wavelengths λ1-λ40, for example, are included in a signal light S1 radiated from the common port 1 through the lens L1 to the multilayer film filter F, the wavelengths λ1-λ4 are passed through the multilayer film filter F and terminated by an optical terminator T1. However, the wavelengths λ5-λ40 are reflected by the multilayer film filter F and outputted from the reflect port P3.
On the other hand, when lights of the wavelengths λ1-λ4 are radiated from the insert port P2 through the lens L2 to the multilayer film filter F, they are transmitted unchanged through the multilayer film filter F to be outputted from the reflect port P3.
Therefore, by conforming the wavelengths transmitted through the multilayer film filter F to those of the lights inputted from the insert port P2 (wavelengths λ1-λ4 for both in the example shown in FIG. 13B), the reject/add filter 1 will acquire both of a function of a wavelength blocker for blocking a part of the wavelengths (wavelengths λ1-λ4) from among the wavelengths λ1-λ40 inputted from the common port P1 and a function of an optical coupler multiplexing (coupling) the wavelengths λ1-λ4 inputted from the insert port P2 with the wavelengths λ5-λ40 inputted from the common port P1 and reflected by the multilayer film filter F.
It is to be noted that in the reject/add filter 921 shown in FIG. 12, the reject/add filter 1 of the basic arrangement shown in FIG. 13A is connected in 3 stages.
The reason why the reject/add filter 921 in the optical add/drop multiplexer 900 shown in FIG. 12 is arranged in 3 stages will now be described referring to FIGS. 14A and 14B.
FIG. 14A shows an example of three reject/add filters 1_1-1_3 similar to the reject/add filter 1 of the basic arrangement shown in FIGS. 13A and 13B connected to compose a three-staged reject/add filter 921 while as shown in FIG. 14A, the respective insert ports of the reject/add filter 1_1 and 1_2 are provided with optical terminators T1 and T2 so that the light does not enter. Also as shown in FIG. 14A, the common port P1 of the reject/add filter 921 is the common port of the reject/add filter 1_1, and the insert port P2 and the reflect port P3 of same are ports of the reject/add filter 1_3.
In order to avoid occurrence of adverse effects due to interferences and the like between the wavelengths λ1-λ4 inputted from the insert port P2 and the wavelengths λ1-λ4 inputted from the common port P1, interception loss should be less than about 40 dB for the wavelengths λ1-λ4 inputted from the common port P1.
As shown in FIG. 14A, the interception losses of the wavelengths λ1-λ4 at the times of output from the reject/add filter 1_1 of the first stage and the reject/add filter 1_2 of the second stage, are about 23 dB and 31 dB, respectively, so that they are insufficient. Therefore, in order to achieve the interception loss of about 40 dB, it is required to have three-staged arrangement as shown in FIG. 14A. In this case the interception loss of 38 dB can be achieved.
FIG. 14B shows examples of an interception characteristic and a transmission characteristic of a block wavelength λB (wavelengths λ1-λ4 in the example of FIG. 14A) when the reject/add filter 921 is used.
While it is made possible to secure enough interception loss by making the reject/add filter 921 have a three-staged arrangement as described above, the transmission (through) losses of the wavelengths λ5-λ40 increase to the contrary. Namely, the transmission losses assume 0.5 dB for the first stage, 1.0 dB for the second stage, and 1.5 dB for the third stage.
It is to be noted that specific examples of the reject/add filter 1 of the basic arrangement shown in FIGS. 13A and 13B include commercially available products such as “4 Skip 1 Filter” manufactured by Fibernett Co., Ltd.
As another example of an optical add/drop multiplexer, there is one provided with at least two variable wavelength selective filters of a first variable wavelength selective filter for performing a branching and inserting operation for a part of light signals from among light signals to be branched and inserted, and a second variable wavelength selective filter for performing a branching and inserting operation for the light signals to be branched and inserted which were unselected by the first variable wavelength selective filter, thereby branching or inserting all of the light signals to be branched and inserted by using a plurality of the variable wavelength selective filters (see e.g. patent document 1).
In this case, Acousto-Optic Tunable Filters (AOTFs) are used as the first and second variable wavelength selective filters.
Also, there is an optical add/drop multiplexer dropping and inserting light signals without using a multiplexer, a demultiplexer, an optical combiner or a splitter by using a channel selective switch operated in a “bar and cross” state (see e.g. patent document 2).
In this case, optical multiplexed channels are received by the first input of the channel selective switch, channels that are selected so as to be added from the associated line terminal apparatus are received by a second input to be bar-connected to a second output. On the other hand, channels that are selected so as to be dropped are bar-connected from the first input to the first output, while channels not selected so as to be dropped are cross-connected to the second output.
Moreover, there is an optical add/drop multiplexer capable of arbitrarily selecting wavelengths of light signals to be dropped or inserted by using an optical circuit to which arbitrary combination of reflected wavelengths and transmitted wavelengths can be set with an external control (see e.g. patent document 3).    (Non-patent document 1) Goji Nakagawa et al. “Photonic Gateway for Metro Network using Acousto-Optic Tunable Filter” [IEICE Technical report Vol. 103 No. 68 Technical Repot of IEICE CS2003-11, p. 13-17]    (Patent document 1) Japanese Patent Application Laid-open No. 11-289296    (Patent document 2) Japanese Patent Application (Translation of PCT Application) No. 11-508428    (Patent document 3) Japanese Patent No. 3401189
Since the three-staged reject/add filter 921 is required as mentioned above in the optical add/drop multiplexer 900 shown in FIG. 12, the transmission loss of the through wavelength 30 is increased.
Also, considering losses of the through wavelength 30 and the drop wavelength 20 due to the power branching performed by the optical coupler 911 within the optical branching portion 910 and the transmission loss of the drop wavelength in the wavelength selective filters 913_1-913—n in the dropping direction, either one or both of the optical amplifier 914 in the dropping direction and the optical amplifier 915 in the through direction may be required in order to compensate for these losses.
Also, the wavelength selective filters 913_1-913—n are required as much as the number of the drop ports. Since the recent data communication demands trend to increase, the number of drop ports is anticipated to increase inevitably. Namely, the number of the wavelength selective filters 913_1-913—n required for a single optical add/drop multiplexer is on an upward trend, so that increases in cost and size are inevitable.
Due to the above-mentioned use of optical amplifiers 914 and 915 as well as the increase in the number of the wavelength selective filters 913_1-913—n, it is anticipated that the cost of the apparatus per single optical add/drop multiplexer will increase and the apparatus will grow in size.